An illumination source is largely characterized by luminous flux (lumen) and spectral power distribution (W/nm). The former is a metric of the perceived brightness whereas the latter determines the color of the light via multiplication with the color matching functions. The color of white light can be expressed both by CIE chromaticity coordinates and by the correlated color temperature (CCT), that is, the temperature of a black-body radiator resulting in a spectrum which, when multiplied by the color matching functions, yields the same color as the original illumination source. For example, an incandescent light bulb has a spectrum corresponding to a CCT of 3200° K. whereas a Xenon camera flash typically has a CCT of 9000° K. The CCT of daylight varies by weather, location and time of the day and year.
The human vision adapts to the illumination so an object with flat reflection spectrum looks white under many different illumination sources. In contrast, a film-based camera is not able to adapt. In a digital camera, the sensor usually has fixed RGB (red, green, blue) filters, post-processing of the raw image data can be used to adjust the white balance to a predefined value, usually expressed in CCT. In particular, in consumer cameras, this process is automated via automatic white balancing (AWB) algorithms, i.e. the white point of the image is adjusted after it has been recorded. These algorithms are often very intricate and advanced but the result is always implemented by adjusting the relative gain in the red, green, and blue channels.